STATOR AND ROTOR LAMINATION ANNEALING BASIC INFORMATION

Stator Lamination Annealing

Semiprocessed lamination sheet is received from the producing mill in the heavily temper-rolled condition. This condition enhances the punchability of the sheet and provides energy for the metallurgical process of grain growth that takes place during the annealing treatment.Annealing of the laminations is done for several reasons. Among them are the following.

Cleaning. Punched laminations carry some of the punching lubricant on their surfaces. This can be a water-based or a petroleum-based lubricant. It must be removed before the laminations enter the high temperature zone of the annealing furnace to avoid sticking and carburization problems. This is done by preheating the laminations in an air or open-flame atmosphere to 260 to 427°C (500 to 800°F).

Carbon Control. Carbon in solution in steel can form iron carbides during mill processing, annealing, and electromagnetic device service. These carbides have several effects on properties—all detrimental. They affect metallurgical processing in the producing mill, degrading permeability and, to some extent, core loss.

They pin grain boundaries during annealing, slowing grain growth.They pin magnetic domain walls in devices, inhibiting magnetization and thus increasing core losses and magnetizing current. If the carbides precipitate during device use, the process is called aging.

Because of these problems, the amount of carbon is kept as low as is practical during mill processing. The best lamination steels are produced to carbon contents of less than 50 ppm. Steels of lesser quality can be produced with up to 600-ppm carbon, but in the United States, 400 ppm is presently a practical upper limit.Laminated cores cannot run efficiently with these high carbon contents, so the carbon is removed by decarburization during annealing.

The annealing atmosphere contains water vapor and carbon dioxide,which react with carbon in the steel to form carbon monoxide.The carbon monoxide is removed as a gas from the furnace.

This process works well for low-alloy steels, but for steels with appreciable amounts of silicon and aluminum, the same water vapor and carbon dioxide provide oxygen that diffuses into the steel, forming subsurface silicates and aluminates.

These subsurface oxides impede magnetic domain wall motion, lowering permeability and raising core loss.

Grain Growth. The grain diameter that minimizes losses in laminations driven at common power frequencies is 80 to 180 μm.As the driving frequency increases, this diameter will decrease. Presently, the temper-rolling percentage and the annealing time and temperature are designed to achieve grain diameters of 80 to 180 μm.

Coating. Laminations punched from semiprocessed steel are uncoated, while those punched from fully processed sheet are typically coated at the steel mill with a core plate coating. This coating insulates laminations from each other to reduce interlamination eddy currents, protects the steel from rust, reduces contact between laminations from burrs, and reduces die wear by acting as a lubricant during stamping.

The semiprocessed steel laminations are also improved by a coating, but economics precludes coating them at the steel mill. Instead, they are coated at the end of the annealing treatment when the laminations are cooling from 566°C to about 260°C (1050°F to about 500°F). The moisture content of the annealing atmosphere is controlled to form a surface oxide coating of magnetite.

This oxide of iron is very adherent and has a reasonably high insulating value. Therefore, it can be used for the same purposes as the relatively expensive core plate coating on the fully processed steel laminations. This magnetite coating is referred to as a blue coating or bluing because blue to blue-gray is its predominant color.

Rotor Lamination Annealing

Sometimes rotors are annealed with the stators, but often they are only given a rotor blue anneal.This is similar to the end of the stator anneal,mentioned previously.

The rotors are heated to about 371°C (700°F) in a steam-containing atmosphere to form a magnetite oxide on their surface. They are then die-cast with aluminum to form conductor bars and end rings. The magnetite oxide prevents adherence of the aluminum to the steel laminations and thereby reduces rotor losses.Stator Lamination Annealing

Semiprocessed lamination sheet is received from the producing mill in the heavily temper-rolled condition. This condition enhances the punchability of the sheet and provides energy for the metallurgical process of grain growth that takes place during the annealing treatment.Annealing of the laminations is done for several reasons. Among them are the following.

Cleaning. Punched laminations carry some of the punching lubricant on their surfaces. This can be a water-based or a petroleum-based lubricant. It must be removed before the laminations enter the high temperature zone of the annealing furnace to avoid sticking and carburization problems. This is done by preheating the laminations in an air or open-flame atmosphere to 260 to 427°C (500 to 800°F).

Carbon Control. Carbon in solution in steel can form iron carbides during mill processing, annealing, and electromagnetic device service. These carbides have several effects on properties—all detrimental. They affect metallurgical processing in the producing mill, degrading permeability and, to some extent, core loss.

They pin grain boundaries during annealing, slowing grain growth.They pin magnetic domain walls in devices, inhibiting magnetization and thus increasing core losses and magnetizing current. If the carbides precipitate during device use, the process is called aging.

Because of these problems, the amount of carbon is kept as low as is practical during mill processing. The best lamination steels are produced to carbon contents of less than 50 ppm. Steels of lesser quality can be produced with up to 600-ppm carbon, but in the United States, 400 ppm is presently a practical upper limit.Laminated cores cannot run efficiently with these high carbon contents, so the carbon is removed by decarburization during annealing.

The annealing atmosphere contains water vapor and carbon dioxide,which react with carbon in the steel to form carbon monoxide.The carbon monoxide is removed as a gas from the furnace.

This process works well for low-alloy steels, but for steels with appreciable amounts of silicon and aluminum, the same water vapor and carbon dioxide provide oxygen that diffuses into the steel, forming subsurface silicates and aluminates.

These subsurface oxides impede magnetic domain wall motion, lowering permeability and raising core loss.

Grain Growth. The grain diameter that minimizes losses in laminations driven at common power frequencies is 80 to 180 μm.As the driving frequency increases, this diameter will decrease. Presently, the temper-rolling percentage and the annealing time and temperature are designed to achieve grain diameters of 80 to 180 μm.

Coating. Laminations punched from semiprocessed steel are uncoated, while those punched from fully processed sheet are typically coated at the steel mill with a core plate coating. This coating insulates laminations from each other to reduce interlamination eddy currents, protects the steel from rust, reduces contact between laminations from burrs, and reduces die wear by acting as a lubricant during stamping.

The semiprocessed steel laminations are also improved by a coating, but economics precludes coating them at the steel mill. Instead, they are coated at the end of the annealing treatment when the laminations are cooling from 566°C to about 260°C (1050°F to about 500°F). The moisture content of the annealing atmosphere is controlled to form a surface oxide coating of magnetite.

This oxide of iron is very adherent and has a reasonably high insulating value. Therefore, it can be used for the same purposes as the relatively expensive core plate coating on the fully processed steel laminations. This magnetite coating is referred to as a blue coating or bluing because blue to blue-gray is its predominant color.

Rotor Lamination Annealing

Sometimes rotors are annealed with the stators, but often they are only given a rotor blue anneal.This is similar to the end of the stator anneal,mentioned previously.

The rotors are heated to about 371°C (700°F) in a steam-containing atmosphere to form a magnetite oxide on their surface. They are then die-cast with aluminum to form conductor bars and end rings. The magnetite oxide prevents adherence of the aluminum to the steel laminations and thereby reduces rotor losses.

ROTOR DESIGN OF DIRECT CURRENT GENERATORS BASIC INFORMATION

Rotor Speeds. Standards list dc generator speeds as high as are reasonable to reduce their size and cost. The speeds may be limited by commutation, maximum volts per bar, or the peripheral speeds of the rotor or commutator.

Generator commutators seldom exceed 5000 ft/min, although motor commutators may exceed 7500 ft/min at high speeds. Generator rotors seldom exceed 9500 ft/min. If the prime mover requires lower speeds than these, generators can be designed for them but larger machines result.

Rotor Diameters.

Difficult commutating generators benefit from the use of large rotor diameters, but diameters are limited by the same factors as rotor speeds. The resultant armature length should be not less than 60% of the pole pitch, because such a small portion of the armature coil would be used to generate voltage.

Direct-current motor speeds must suit the application, and often the rotor diameter is selected to meet the inertia requirements of the application. Core lengths

may be as long as the diameter. Such motors are usually

force-ventilated.

Number of Poles and Other Rotor Design Factors.

The rotor diameter usually fixes the number of main poles. Typical pole pitches range from 17.5 to 20.5 in on medium and large machines. When a choice is possible, high-voltage generators use fewer poles to allow more voltage space on the commutator between the brush arms.

However, high-current generators need many poles to permit more current carrying brush arms and shorter commutators. Commutators for 1000 to 1250 A/(brush arm) (polarity) are costly, and lower values should be used where existing dies will permit.

The main-pole air-gap flux density Bg is limited by the density at the bottom of the rotor teeth. The reduced taper in the teeth of large rotors permits the higher gap densities.

Ampere conductors per inch of rotor circumference (q) is limited by rotor heating, commutation, and, at times, saturation of commutating poles. The commutator diameter is usually about 55% to 85% of the rotor diameter, depending on the sizes available to the designer, the peripheral speed, and the resulting single clearances.

Heating may also limit the choice. Brushes and brush holders are chosen from designs available to limit the brush current density to 60 to 70 A/in2 at full load, to obtain the needed single clearance, and to obtain acceptable commutator heating.

MAIN GENERATOR TYPES BASIC TUTORIALS

The two main types of generator are ‘turbo’ or cylindrical-rotor and salient-pole generators. Both these types are synchronous machines in which the rotor turns in exact synchronism with the rotating magnetic field in the stator.

The largest generators used in major power stations are usually turbo-generators. They operate at high speeds and are usually directly coupled to a steam or gas turbine.

The general construction of a turbo-generator is shown in Fig. 5.1. 

The rotor is made from solid steel for strength, and embedded in slots within the rotor are the field or excitation windings. The outer stator also contains windings which are located in slots, this is again for mechanical strength and so that the teeth between the slots form a good magnetic path.

Most of the constructional features are very specialized, such as hydrogen cooling instead of air, and direct water cooling inside the stator windings, so only passing reference is made to this class of machines in the following descriptions.

More commonly used in smaller and medium power ranges is the salient-pole generator. An example is shown in Fig. 5.2. Here, the rotor windings are wound around the poles which project from the centre of the rotor.

The stator construction is similar in form to the turbo-generator stator shown in Fig. 5.1. Less commonly used are induction generators and inductor alternators.

Induction generators have a simple form of rotor construction as shown in Fig. 5.3, in which aluminium bars are cast into a stack of laminations. These aluminium bars require no insulation and the rotor is therefore much cheaper to manufacture and much more reliable than the generators shown in Figs 5.1 and 5.2.

The machine has characteristics which suit wind turbines very well, and they also provide a low-cost alternative for small portable generators.

Inductor alternators have laminated rotors with slots, producing a flux pulsation in the stator as the rotor turns. These machines are usually used for specialized applications requiring high frequency.

GENERATOR ROTOR CONSTRUCTION BASIC INFORMATION

It has already been noted that the construction of a turbogenerator is very specialized and the rotor for these machines are not dealt with here. However, even within the class of salient-pole generators, quite different forms of rotor construction are used, depending upon the size.

Generators rated up to about 500 kW use rotor laminations which are stamped in one piece. In larger machines the poles are made separately from stacks of laminations, and each pole is keyed using a dovetail arrangement onto a spider which is mounted on the rotor shaft.

In large high-speed machines the poles can be made from solid steel for extra strength and to reduce mechanical distortion; these solid poles are screwed to the shaft, as shown in the large 4-pole machine in Fig. 5.23.

Fig. 5.23 Large salient-pole rotor (courtesy of Brush Electrical Machines)

The nature of the rotor coils also depends upon the size of the machine. Because the ratio of surface area to volume is larger in the coils of small generators, these are easier to cool.

Generators rated above about 25 kW therefore use a ‘layer-wound’ coil in which each layer of the coil fits exactly into the grooves formed by the layer below.

Rectangular cross section wire can be used to minimize the coil cross section. The simplest and cheapest way to make the coils, often used in smaller machines, is to wind them in a semi-random way.

In either case, the coils are impregnated after winding like the stator windings to give extra mechanical strength and to improve the heat transfer by removing air voids within the coil. The coils are under considerable centrifugal stress when the rotor turns at full speed, and they are usually restrained at both ends of the pole by bars, and by wedges in the interpole spaces, as shown in Fig. 5.23.

STATOR AND ROTOR CONSTRUCTION OF AC GENERATORS

Stator Construction

Armature cores are built up of thin laminations, produced as either segments or complete rings, depending on the size of the generator. Successive layers or groups of layers of the segmented laminations are staggered to minimize the effect of the joints in the magnetic circuit.

The core is clamped between pressure plates and fingers to support it with sufficient pressure to prevent undue vibration of the laminations. Especially in long cores, the clamping arrangement may include some provision to compensate for compacting of the core after initial assembly.

The armature windings are fitted tightly in the slots and secured radially by wedges driven into suitable notches at the air gap end of the slots. It is necessary that the stator coil ends be able to resist the abnormal forces associated with short circuits.

A supporting structure may be employed for this purpose. There are many variations of support design; most of them provide filler blocks between the coil sides, strategically located to transmit the circumferential forces from coil to coil, and additional structure to counteract the radial forces.

Coil supports ordinarily are designed to suit the need of a particular machine. Large 2-pole machines require a quite elaborate structure; the combination of large short-circuit currents and coil ends inherently flexible because of their long length makes these machines particularly susceptible to coil-end movement.

Low-speed machines with stiffer coil ends require less support; in the smallest ratings the coils may be capable of withstanding the short-circuit sources without any additional support.

Stator frames, sometimes called casings, are commonly fabricated from structural steel, designed to support the core in alignment with the rotor and to suit the ventilating scheme used. In large machines with 2-pole or sometimes 4-pole construction, the stator core is mounted on springs to isolate core vibration from the machine frame.

Rotor Construction

The pole pieces of salient-pole alternators may be built up of steel laminations, both as manufacturing convenience and a means of limiting the loss in their air gap surfaces due to pulsations in air gap flux. The field coils, wound directly on the poles or preformed and then mounted on the poles, are suitably insulated from the poles for the voltages associated with normal and transient operation.

The pole-and-coil assembly is bolted, dovetailed, or otherwise attached to the rotor body. It is the limitation of this attachment which usually dictates when round-rotor construction must be used rather than salient-pole construction.

The rotor body for a salient-pole machine may be a solid forging or assembly of heavy steel plates, for high speed designs, or a spider-and-rim assembly for low-speed designs. The shaft may be integral with the body, as in the case of a forging, or may be bolted to or inserted into the body.

When the spider-and-rim construction is used, the entire assembly may be an integral weldment or casting, or the rim may be separate from the spider, as in the case of large waterwheel-driven generators.

A common construction for this latter case is a rim built up of thin steel laminations, assembled around a cast or fabricated spider, bolted together between steel end plates and keyed to the spider.

The rotor of a round-rotor machine is cylindrical in shape with axial slots provided in its body for the field coils. The body is usually a steel forging with integral shaft ends. In special applications, other constructions may be used, with this same general configuration.

The field coils are wound in axial slots in the rotor body, held in place by heavy slot wedges and by retaining rings over the coil ends.

Rotors are designed for operation at overspeeds, which depend on the characteristics of the prime mover. The overspeed limit (the speed above which the unit is no longer capable of safe operation) may be as low as 20% for a steam-turbine-driven unit or as high as 125% for some adjustable-blade, axial-flow hydraulic turbine-driven units.